- https://developer.mozilla.org/zh-CN/docs/Web/JavaScript/Reference/Statements/for...of
- https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Statements/for...of
- https://hacks.mozilla.org/2015/04/es6-in-depth-iterators-and-the-for-of-loop/
- http://www.infoq.com/cn/articles/es6-in-depth-iterators-and-the-for-of-loop